1. Field of the Invention
The present invention relates to a developing apparatus which forms a visible image by developing an electrostatic latent image formed on an image bearing member through an electrophotographic printing process or an electrostatic recording process. In particular, the present invention relates to an image forming apparatus such as a copier, a printer, a recorded image display apparatus, and a facsimile, the image forming apparatus including a developing apparatus which uses a dual-component developer including a toner and a carrier.
2. Description of the Related Art
Conventionally, an image forming apparatus employing an electrophotographic printing process or an electrostatic recording process, in particular, a color image forming apparatus configured to form a full color image by the electrophotographic printing process mostly includes a developing apparatus which uses a dual-component developer obtained by mixing a toner and a carrier in view of chromogenic properties and color blend properties.
As is well known, a developing method using the dual-component developer is a method of forming an image by electrically charging toner particles by triboelectric charging of carrier particles and the toner particles, and causing the electrically charged toner particles to electrostatically adhere to an electrostatic latent image. In such a dual-component developing process, in order to highly stably provide images while suppressing variation in density, it is important to stabilize a toner charge amount (hereinafter referred to as “triboelectricity”), and for this stabilization, it is necessary to equalize a toner density distribution in the developing apparatus. In general, the triboelectricity is liable to be influenced by a toner density, specifically, tends to increase in absolute value in accordance with a decrease of the toner density, and to decrease in absolute value in accordance with an increase of the toner density.
In view of this, in the conventional developing apparatus, when a toner is consumed along with development and the toner density of the developer decreases, a toner is replenished by an amount of compensating the consumed toner, and agitated. In this way, the toner density is controlled and maintained at a constant level.
However, along with use over a long period of time, at the time of development, in a state in which a toner is consumed, a developer collected from a developing sleeve to the developing apparatus may be resupplied to the developing sleeve at a partially non-uniform toner density without being sufficiently mixed with a developer in the developing apparatus. As a result, there arises a problem of the decrease of the toner density.
As a countermeasure for the problem described above, as described below, there has been proposed a structure in which, at the time of development, the developer reduced in toner density by consumption of a toner is prevented from being resupplied to the developing sleeve immediately after being collected in the developing apparatus. Specifically, there has been proposed a developing apparatus separately including a supply chamber configured to supply a developer to the developing sleeve and a collection chamber configured to collect the developer from the developing sleeve (Japanese Patent Application Laid-Open No. H05-333691). As illustrated in FIG. 11, this developing apparatus 101 includes a supply chamber 102 provided in an upper portion and a collection chamber 103 provided in a lower portion of the developing apparatus 101. The developing apparatus 101 further includes two upper and lower screws: a first conveying screw 104 and a second conveying screw 105, which are opposite to each other in a conveying direction, for circulating the developer between the collection chamber 103 and the supply chamber 102. While being circulated, the developer is supplied from the upper supply chamber 102 to a developing sleeve 106, and a photosensitive member 108 is subjected to development. Meanwhile, after completion of the development, the developer is collected from the developing sleeve 106 into the lower collection chamber 103. With this, the after-development developer reduced in toner density is not immediately resupplied to the developing sleeve 106. Thus, problems of partial non-uniformity in toner density and a decrease of the toner density are alleviated.
However, even with use of such a developing apparatus, the problems of partial non-uniformity in toner density, a decrease of the toner density, and the like have not yet been alleviated in a case where a toner consumption is large as in a case of forming an image of a high coverage rate.
In the collection chamber, a toner replenished in the developer and the developer collected from the developing sleeve are merged, and then agitated and conveyed. Thus, a level of the developer tends to be higher toward a downstream side in the conveying direction in the collection chamber. When the level of the developer becomes higher, the developer is conveyed to the supply chamber without being sufficiently agitated by the conveying (agitating) screw provided in the collection chamber. Thus, a part of the developer, which is collected in a region on the downstream side in the conveying direction in the collection chamber, is liable to be insufficiently agitated and mixed with another developer subjected to toner replenishment. Thus, those developers are liable to be transferred to the supply chamber without being mixed with each other. There is no problem in a case of forming an image of a low coverage rate. However, when the toner is insufficiently agitated in the case of forming an image of a high coverage rate, there remains a risk that the developer is transferred to the supply chamber without being equalized in density and the developer in uneven density may be supplied as it is to the developing sleeve.
As a countermeasure for the problem described above, there has been proposed a developing apparatus further including, in addition to the second conveying screw in the collection chamber, a third conveying screw configured to convey the developer in a direction opposite to that of the second conveying screw (Japanese Patent No. 3,127,594). As illustrated in FIG. 12, when a third conveying screw 107 is provided, a part of the developer, which stagnates on a rear side close to a communication portion through which the developer is transferred from the collection chamber 103 to the supply chamber 102, can be forced back in the direction opposite to the conveying direction of the second conveying screw 105. As a result, the level of the developer can be equalized, and an effect of agitating the dropped developer can be enhanced.
As in the structure of Japanese Patent No. 3,127,594, from a part of the developer, which stagnates in the communication portion between the collection chamber to the supply chamber (developing chamber), another part of the developer, which overflows on the third conveying screw side, can be forced back by the third conveying screw. As a result, it is possible to suppress rise of the level of the developer on the downstream side in the conveying direction, and the effect of equalizing the level of the developer can be obtained to some extent. However, as disclosed in Japanese Patent No. 3,127,594, in the structure in which the level of the developer is merely equalized, a part of the developer reduced in toner density and collected in the collection chamber may be immediately conveyed to the communication portion communicating to the supply chamber without being sufficiently agitated. Thus, in order to solve the problems described above, the developer reduced in toner density and collected in the collection chamber needs to be effectively mixed with the developer subjected to toner replenishment before being conveyed to the communication portion communicating to the supply chamber.